1. Field of the Invention
The invention generally relates to a take-off control apparatus and method for a vehicle equipped with an automatic transmission. More particularly, the invention relates to a take-off control apparatus and method for a vehicle, which executes a neutral control.
2. Description of the Related Art
An automatic transmission mounted in a vehicle is connected to an engine via a fluid coupling such as a torque converter provided in the automatic transmission, and includes a gear change mechanism that provides a plurality of power transmission paths. This automatic transmission is structured so as to automatically switch the power transmission path, i.e., the gear ratio (drive speed), based on the accelerator opening amount and the vehicle speed, for example. A vehicle having an automatic transmission is typically provided with a shift lever which is operated by a driver into any one of various shift positions (such as REVERSE, NEUTRAL, DRIVE). Automatic shift control is performed when the shift lever is in one of these shift positions (usually in a drive position or forward-drive range).
When a vehicle having this type of automatic transmission is stopped with the shift lever in a position corresponding to a forward-drive range, such as DRIVE, driving force from the idling engine is transmitted to the transmission via the torque converter and then to the wheels, resulting in a phenomenon known as “creeping.” Creeping is extremely useful under certain conditions. For example, it facilitates smooth take-off of a vehicle when starting from a standstill on an incline. When the driver wants a vehicle that is stationary on a flat road to remain in one spot, however, creeping is unnecessary and must be suppressed by operating the brake. That is, the brake is used to suppress the creeping force from the engine, and the fuel efficiency of the engine decreases by a corresponding amount.
To improve fuel efficiency, therefore, it has been proposed to put a transmission that is in the forward-drive range into a neutral state, including a semi-neutral state, that resembles NEUTRAL, while still in the forward-drive range and the vehicle is being held stationary by the brake pedal being depressed so as to operate the brake and the accelerator being almost completely closed.
Many of these kinds of so-called neutral control technologies and technologies for controlling a brake of a vehicle during the transition from a stationary state to a take-off state are known.
JP(A) 10-196782 discloses a creep prevention device with hill hold for an automatic transmission, which cancels a creep prevention control (i.e., neutral control) and hill hold control at an appropriate timing and in such a manner that shock is reduced. This creep prevention device is used with an automatic transmission that inputs rotation of an engine via a fluid power transmitting device and changes the rate and/or direction of that rotation as appropriate according to a speed selected by selectively hydraulically operating a plurality of friction apply elements, before outputting it. In order to reduce the transmission of creep torque generated by the fluid power transmitting device while a stopped vehicle is maintained in a braked state, the creep prevention device places a friction apply element, from among the plurality of friction apply elements, which is applied for take-off when the vehicle is stopped, in a slip state by reducing the hydraulic fluid pressure of that friction apply element. The creep prevention device also hydraulically operates a friction apply element, from among the plurality of friction apply elements, capable of holding a vehicle on a hill. The application of this friction apply element prevents the output shaft of the transmission from rotating when creep prevention control is being executed. This creep prevention device is structured so as to increase the hydraulic fluid pressure of the friction apply element for take-off at a predetermined gradient when the creep prevention control is to be cancelled by a brake of the vehicle being released from an applied state. The predetermined gradient is set such that the period of time necessary for reapplying the friction apply element (i.e., the time that it takes for the friction apply element to be completely applied from a slip state) is maintained at an appropriate period of time.
The creep prevention device disclosed in JP(A) 10-196782 places the friction apply element in a slip state by reducing the hydraulic fluid pressure of the friction apply element, from among the plurality of friction apply elements, which is applied for take-off when the vehicle is stopped, when reducing the transmission of creep torque generated by the fluid power transmitting device while the vehicle is stopped with the brake applied. At the same time, the creep prevention device also has a hill hold function which prevents the vehicle from moving while in the creep prevention state on a hill. This function is achieved by the device hydraulically operating a friction apply element, from among the plurality of friction apply elements, capable of holding a vehicle on a hill, which prevents the output shaft of the transmission from rotating. When the creep prevention control is to be cancelled by a brake of the vehicle being released from an applied state, the hydraulic fluid pressure of the friction apply element for take-off is increased at a predetermined gradient. The predetermined gradient is set such that the period of time necessary for reapplying the friction apply element (i.e., the time that it takes for the friction apply element to be completely applied from a slip state) is maintained at an appropriate period of time. As a result, even if there is dispersion in the coefficient of friction of the friction apply element for take-off or if that coefficient of friction changes over time, the period of time required for reapplying that friction apply element (i.e., the time that it takes for the friction apply element to be completely applied from a slip state) is able to be reliably maintained at an appropriate period of time. Accordingly, the possibility of a large shock being generated due to the friction apply element for take-off being reapplied too quickly is able to be eliminated. At the same time, it is also possible to avoid a delay in the completion of reapplication of the friction apply element for take-off and the release of the friction apply element capable of holding a vehicle on a hill before that reapplication is complete, which would result in the shock generated by the reapplication of the friction apply element for take-off being transmitted directly to the wheels, or in the hill hold function being unable to be replaced by the use of creep torque.
When the neutral control is cancelled using the creep prevention device disclosed in JP(A) 10-196782, application of the friction apply element for take-off (i.e., cancellation of the neutral control) can be reliably completed after the appropriate period of time has passed. However, because the appropriate period of time corresponds to dispersion in the coefficient of friction of the friction apply element for take-off and the like, this is nothing more than determining completion of cancellation of the neutral control (i.e., reapplication of the friction apply element for take-off) based on the engine speed and the temperature of the hydraulic fluid of the automatic transmission. That is, the technology disclosed in the foregoing publication does not determine the completion timing of the cancellation of the neutral control taking into account any factors indicative of the state of the vehicle other than those mentioned above (i.e., the engine speed and the temperature of the hydraulic fluid in the automatic transmission).